The present invention relates to a die-casting method and machine for manufacturing a die-cast product usable as not only structural members but also functional members by virtue of its suppressed cast defects such as internal porosities or blowholes.
In a conventional die-casting method, molten aluminum or molten aluminum alloy (hereinafter correctively referred to as xe2x80x9cmolten metalxe2x80x9d) poured into a sleeve of a die-casting machine is forcibly injected into a cavity defined in dies by a plunger in the sleeve. While most of gases, such as air or water vapor, residing in the cavity are purged from the cavity in conjunction with the injection of the molten aluminum, a part of the gases can be undesirably left in the cavity after the completion of the injection of the molten aluminum alloy. Particularly, a die assembly designed for a specific product having a very thin thickness or a complicated configuration can involve narrowed portions constraining fluid flow, and thereby it is difficult to achieve complete exclusion of gases residing in its die cavity.
During a cooldown/solidification stage of the injected molten aluminum in the dies, the residual gases in the cavity are took in the molten aluminum and incorporated into the resulting die-cast product as a factor of casting defects such as internal porosities or blowholes. As a result, the obtained die-casting products have suffered from inferiority in mechanical properties such as strength or elongation, resulting in unfavorable evaluation for being inadequate to use as functional components, such as a scroll, piston, cylinder block, connecting rod or suspension member. If such casting defects caused by the residual gases are suppressed, the die-casting method will have enlarged range of applicable fields with its intrinsic excellent productivity.
There has been known a vacuum die-casting method as one of techniques intended to eliminate the adverse effect of the residual gases. In the vacuum die-casting method, a die cavity is evacuated prior to a forcible injection of molten aluminum in order to remove air from the cavity. However, in this method, the internal pressure of the cavity has a limited vacuum ranging from 200 to 500 millibar and it is practically impossible to obtain further reduced vacuum, because some ambient air enters in the cavity through a gap between mating faces of dies and another ambient air is additionally introduced in a sleeve of a die-casting machine during the operation of pouring the molten aluminum into the sleeve. Thus, even though a product obtained from the vacuum die-casting method has a reduced volume of incorporated air as compared with products from another conventional die-casting method, it is still involved with the casting defects such as internal porosities caused by the incorporated gases, and is thereby quite inadequate to use as functional components.
An oxygen die-casting method has been developed to eliminate the disadvantage of the vacuum die-casting method (see Japanese Patent Publication No. 50-21143). In the oxygen die-casting method, oxygen gas is filled in a die cavity with a pressure of atmospheric pressure or more to replace gases in the cavity by the oxygen gas. Thus, an excessive part of the supplied oxygen gas is blown out of the cavity through a gap between mating faces of dies and a pour opening for pouring molten aluminum therethrough to prevent ambient air from entering in the cavity through the gap and the pour opening. The supplied oxygen gas remaining in the cavity reacts with the molten aluminum to form a fine structure of Al2O3 dispersed over the resulting die-cast product without any adverse effect on the properties of the product.
However, even by supplying the oxygen gas into the cavity with the pressure of atmospheric pressure or more, it is difficult to completely remove air from the cavity. Generally, a die cavity having a complicated configuration leads to an increased volume of residual air therein. More specifically, the die cavity having a complicated configuration involves a narrowed portion incapable of receiving therein the supplied oxygen gas, and the narrowed portion keeps gases such as air and water vapor residing therein without replacing the gases by the supplied oxygen gas. These residual gases are incorporated into the resulting die-cast product as a factor causing the casting defects.
The residual air in the cavity as a factor causing the casting defects can be effectively replaced by oxygen gas by injecting the oxygen-gas simultaneously with the evacuation of the cavity (Japanese Patent Publication No. 57-140). However, even if the oxygen gas is injected in synchronous with the evacuation of the cavity, water is not effectively removed from the cavity. In fact, a die-cast product obtained from this technique is still involved with the cast defects caused by the residual gases. A technique of injecting oxygen gas after the evacuation of the cavity has been also known (Japanese Patent Publication No. 1-46224). However, this technique cannot sufficiently suppress the casting defects caused by the residual gases, because the inner pressure of the die cavity is maintained merely at a vacuum ranging from about 200 to 400 millibar.
In view of the above circumstance, the inventors has developed a die-casting method comprising the steps of evacuating a die cavity to a vacuum of 100 millibar or less, then injecting a reactive gas such as oxygen gas into the cavity, and initiating a forcible injection of molten aluminum alloy at a time the internal pressure of the cavity is increased to atmospheric pressure or more (Japanese Patent Application No. 11-154566). Evacuating the cavity to the vacuum of 100 millibar or less accelerates vaporization of water from a release agent attached on the inner surfaces of dies. Subsequently supplying the reactive gas to the evacuated cavity allows the reactive gas to be spread all over the interior of the dies, so as to effectively purge the residual air and the water from the release agent as well as other undesirable gases in the cavity. Thus, a die-cast product can be obtained with a significantly reduced volume of incorporated gases, and thereby the casting defects caused by the residual gases can be suppressed. In addition, the die-cast product can be heated without generation of blisters caused by the residual gas. This allows the die-cast product to be improved in its mechanical properties through a heat treatment such as a T6 treatment.
Most of the reactive gas supplied to the cavity reacts with the molten aluminum ally injected into the cavity to form a fine structure of Al2O3 dispersed over the resulting product, and a part of the reactive gas is pushed out of the cavity by the molten aluminum alloy forcibly injected into the cavity. However, depending on the configuration of an intended die-cast product, a part of the reactive gas can be left in the cavity after the completion of the injection of the molten aluminum alloy. Then, the residual reactive gas is undesirably incorporated into the die-cast product without effective reaction with the molten aluminum alloy or in an unreacted state. For instance, in dies intended to manufacture a die-cast product having a complicated configuration, its cavity is typically designed to have a configuration in which a metal flow channel are branched into plural channels and the plurality of channels are jointed together. This junction area creates a dead-end-like portion where a path for pushing out the reactive gas therethrough is clogged by the respective metal flows in the jointed channels to trap the reactive gas.
A system operable to evacuate a die cavity by opening a bypass passage in fluid communication with the cavity (Japanese Patent Publication No. 1-46224) has a limited vacuum ranging from 200 to 400 millibar in the cavity for the following reasons, resulting in a substantial volume of residual air and insufficient purge of the reactive gas in an unreacted state, which leads to insufficient reduction of gases incorporated in a die-cast product.
First, the system is operated to evacuate gases directly from the cavity. Thus, it is required to spend substantial time for evacuating gases from sleeve and runner regions through narrow gates. Further, the system is operated to evacuate gases initially from the cavity. Thus, the resultingly reduced pressure facilitates vaporization of a sleeve lubricant and then sucks the vaporized sleeve lubricant into the cavity, resulting in increased humidity in the cavity. This high humidity can provide water capable of reacting with molten aluminum alloy injected into the cavity to form hydrogen to be incorporated into a die-cast product. A part of the sleeve lubricant can also be sucked into the cavity in a liquid state to cause contamination in the cavity.
Secondly, the system includes an evacuation device adapted to be selectively brought into fluid communication with the cavity through a valve device. That is, if the evacuation operation is continued until the completion of the operation of injecting the molten aluminum alloy, the molten metal can run in the evacuation device through a gap of the opened valve. Thus, it is required to terminate the evacuation of the cavity by shutting the valve before the molten aluminum alloy reaches the valve. As a result, the residual gases cannot be sucked until the completion of the operation of injecting the molten aluminum alloy, and the reactive gas in an unreacted state tends to be left in the cavity.
Further, the evacuation operation and the reactive-gas injection operation are performed by use of a common opening. In other words, it is impossible to inject the reactive gas simultaneously with the evacuation of the cavity, and thereby the reactive gas will be injected after terminating the evacuation of the cavity. Thus, after the termination of the evacuation, some ambient air can enter into the cavity through a gap between mating faces of the dies and tends to remain therein. In addition, since the opening for injecting the reactive gas is provided only in the cavity, the sleeve cannot be filled with the reactive gas, which undesirably allows ambient air to enter into the cavity through a gap between the sleeve and the tip of the plunger.
The present invention has been made to solve the aforementioned problems. It is therefore an object of the present invention to provide an improved vacuum-oxygen die-casting method capable of utilizing both advantages of the conventional vacuum and oxygen die-casting methods and obtaining a die-cast product usable as not only structural members but also functional members, by re-evacuating a die cavity through an overflow region and a runner or sleeve of a die-casting machine during an operation of forcible injecting molten aluminum to discharge a reactive gas in an unreacted state from the cavity so as to achieve a significantly reduced volume of gases incorporated into the die-cast product as compared with conventional die-casting products.
In order to achieve this object, according the present invention, there is provided a die-casting method comprising the steps of evacuating a cavity defined by dies of a die-casting machine to provide therein a vacuum of 100 millibar or less during a first period, injecting a reactive gas from a sleeve of the die-casting machine into the cavity during a second period which has a partial overlap period with the first period and follows the overlap period, so as to increase the inner pressure of the cavity to atmospheric pressure or more, pouring a molten aluminum alloy into the sleeve while keeping the injection of the reactive gas, and subsequently moving a plunger in the sleeve forward to forcibly inject the molten aluminum alloy from the sleeve into the cavity while re-evacuating the cavity.
Preferably, the evacuation is performed at a suction speed of 500 millibar/second or more. The cavity may be evacuated through an evacuation passage opened to a runner region in dies of a die-casting machine, optionally in combination with an evacuation passage opened to an overflow region in the dies. The reactive gas such as oxygen gas is injected into the cavity during the time-period which has the partial simultaneous period with the evacuation period and follows the simultaneous period, so as to increase the inner pressure of the cavity to atmospheric pressure or more. In this reactive-gas injection operation, it is preferable to inject a dehumidified reactive gas to keep the cavity in humidity of 15% RH or less. The reactive-gas injection operation may be terminated before the plunger is moved forward or may be continued until the completion of an entire die-casting operation.
Molten aluminum alloy to be cast is poured from a pour opening of the sleeve to the interior of the sleeve and then forcibly injected into the cavity by the forward movement of the plunger. Preferably, during this molten-aluminum-alloy injection operation, the plunger is temporarily stopped just after the plunger tip passes over the pour opening of the sleeve.
The re-evacuation operation may be initiated after the completion of the operation of pouring the molten aluminum alloy, and continued until the completion of the entire die-casting operation. During the re-evacuation operation, the cavity is evacuated through the gas passage opened to the overflow region, optionally in combination with the gas passage opened to the runner region.
The present invention also provides a die-casting machine suitable for the above method. The die-casting machine comprises: an evacuation device having a discharge passage opened to a runner region which guides molten aluminum alloy poured into a sleeve to a cavity defined by dies, and a gas passage opened to an overflow region in the dies; and a reactive-gas supply device having a gas supply passage opened to a gas inlet provided in the sleeve closer to the dies than a pour opening for pouring the molten aluminum alloy into the sleeve therethrough.
Preferably, a chill vent is provided between the opening of the gas passage and the overflow region, to prevent the molten metal from leaking to the evacuation device. An applicable technique for assuring air-tightness between the cavity and ambient air may include a packing interposed between respective mating faces of stationary and movable dies of the die-casting machine with surrounding the cavity, and a groove which is formed in at least one of the mating faces of the stationary and movable dies with surrounding the cavity and is in fluid communication with the evacuation device. Further, in order to assure air-tightness for an ejector pin slidably inserted in a pin insertion hole which is penetratingly provided in the movable die and opened to the cavity, it is effective to provide a sealing device between the inner surface of the pin insertion hole and the outer surface of the ejector pin.
The gas passage opened to the overflow region may be branched into a discharge passage selectively brought into fluid communication with the evacuation device and an air supply passage selectively brought into fluid communication with a compressed-air supply device, in order to use the gas passage commonly for evacuating the cavity and supplying a compressed air. A pressure gage for measuring the internal pressure of the cavity and a hygrometer for measuring humidity in the cavity may be provided at the gas passage. Preferably, a drier is interposed in the gas supply passage extending from the reactive-gas supply device to the gas inlet of the sleeve.
In the conventional die-casting methods, gases contained in a die-cast product are derived from a residual air in a die cavity. The volume of the residual air can be significantly reduced by the vacuum method or the oxygen die-casting method. However, even if the residual air in the cavity is reduced, a die-cast product obtained from such methods is ineluctably involved with the casting defects caused by the residual gases. In the vacuum-oxygen die-casting method proposed by Japanese Patent Application No. 11-154566, the internal pressure of the cavity is reduced down to the vacuum of 100 millibar or less during the evacuation operation to facilitate vaporization of water from a release agent or the like, and then the reactive gas is supplied to the evacuated cavity to distribute the reactive gas all over the cavity. By supplying the reactive gas to increase the inner pressure of the cavity to atmospheric pressure or more, ambient air is prevented from entering into the cavity, and the water vapor from the release agent is advantageously effused to outside.
In order to depressurize the cavity to the vacuum of 100 millibar or less during the evacuation operation, it is required to provide an airtight structure for the mating faces of the dies, the pour opening and the overflow region. The airtight structure also acts to reliably hold the injected reactive gas in the cavity during the subsequent reactive-gas injection operation so as to keep the cavity in an oxygen atmosphere having a pressure of atmospheric pressure or more. The injected reactive gas does not react altogether with the molten aluminum alloy to form Al2O3, but a part of the reactive gas remains in an unreacted state in the cavity. The unreacted reactive gas is pushed into the overflow region by the molten aluminum alloy forcibly injected into the cavity. However, depending on the configuration of the die cavity, a path for pushing out the unreacted reactive gas can be clogged by the metal flows. In this case, a part of the unreacted reactive gas will be incorporated into a die-cast product. This tendency is strongly developed in a die cavity having a configuration in which metal flow channels are complicatedly branched and jointed.
In the present invention, the gas passages opened to the overflow and runner regions are selectively brought into fluid communication with the evacuation device to evacuate the reactive gas from the cavity during the molten-aluminum-alloy injection operation. Thus, the volume of the unreacted gas incorporated in the molten aluminum alloy is significantly reduced. Further, the evacuation is performed through the gas passage opened to the overflow region. This allows the reactive gas to be injected simultaneously with the evacuation operation, and allows the unreacted reactive gas to be continuously discharged until the completion of the molten-aluminum-alloy injection operation. Thus, the volume of the residual unreacted gas in the cavity will be significantly reduced. During the above evacuation operation, the runner region may be additionally evacuated through the gas passage opened thereto.
Since the unreacted reactive gas is sucked based on evacuation, paths in fluid communication with the evacuating device to evacuate the unreacted reactive gas from the cavity are quickly established before such paths are clogged by the metal flows. Thus, the unreacted reactive gas is not confined in the paths. Further, the runner region can be evacuated just before the molten aluminum alloy enters in the runner region.
In this respect, the technique of directly evacuating an unreacted reactive gas from a die cavity through the bypass passage opened to the cavity, (Japanese Patent Publication 1-46224) cannot achieve the vacuum of 100 millibar or less during the evacuation operation. Thus, it is difficult to prevent the entry of ambient air and the remanence of the unreacted reactive gas, and consequently a die-cast product having a complicated configuration is often involved with the casting defects caused by the unreacted reactive gas.